The Influence of the Dimerisation on the Stoichiometry of the Active Center in Ferredoxin from Clostridium pasteuranium.

Determinations of the molcular weight by aid of gel filtration demonstrate that ferredoxin from Clostridium pasteurianum can exist as well in a monomeric (M_r = 6000) as in a dimeric form (M_r= 12000). The dimerisation takes place in the presence of oxygen. Using exactly anaerobic conditions pure monomeric material can be prepared; using aerobic conditions, however, pure dimeric material or a mixture of monomers, dimers, and apoferredoxins of higher molecular weights appear. Comparing the analytical data of the active center one finds eight atoms of iron and labile sulfur in monomeric ferredoxin, whereas four atoms of iron and labile sulfur are found in the subunit of dimeric ferredoxin. Results of the determination of the Hg-content in mersalyl-apoferredoxin prepared from a dimeric sample and of the titration of free SH-groups in monomers and dimers can be explained assuming that four cysteinyl residues per subunit participate in disulfide bridges in dimeric ferredoxin. That means, dimerisation is accompanied by a loss of iron and labile sulfur and therefore by a decrease of the absorption coefficient ε₃₉₀ = 30000 M^-l × cm^-1 to ε₃₉₀ = 15000 M^-1 × cm^-1 related to one subunit with the molecular weight of 6000. The ratio of absorbances A₃₉₀: A₂₈₀ seems to remain unchanged. In the phosphoroclastic system the biological activity of the dimeric ferredoxin decreases by a factor of two referring to one subunit. [ABSTRACT FROM AUTHOR]